A common kind of ring armature electroacoustic transducer comprises elements which are coaxial about a common vertical axis and which are (a) an annular permanent magnet, (b) a horizontal mechanically resilient armature in the form of a ring or annulus having its outer margin magnetically coupled to the magnet, (c) a lower pole piece with an annular upper end disposed below the inner margin of the armature to be spaced therefrom by a gap known as the "main gap", the pole piece providing a low reluctance magnetic flux path from such gap to part of such magnet displaced from its coupling with the armature, (d) a wire wound coil inductively coupled with such lower pole piece, and (e) a diaphragm mechanically coupled with the armature. The magnet in such a structure produces d.c. magnetic flux which can be considered as flowing from the magnet, radially inward through the ring armature, down through the lower pole piece to the bottom of the magnet and then up through the magnet to its place of beginning. When the transducer is used as a receiver, a.c. electric signal variations in the coil modify the d.c. flux to produce a.c. magnetic flux variations ("a.c. flux") which in turn cause the armature and coupled diaphragm to vibrate and generate sound vibrations corresponding to the signal variations.
In eary versions of such transducer as exemplified by that disclosed in U.S. Pat. No. 2,170,571 issued Aug. 22, 1939 in the name of E. E. Mott, the d.c. flux in the main gap below the inner margin of the armature is not complemented by any d.c. flux in a gap above that inner margin. That is, the magnetic circuit of such transducer is a single magnet, single gap circuit. Later, and as taught in Mott's U.S. Pat. No. 2,249,160 issued July 15, 1941, it was realized that a substantial increase in the "force factor" could be obtained if an auxiliary magnet were added to produce d.c. flux in such an upper or auxiliary gap, force factor being generally defined as the ratio of the mechanical force produced to the magnitude of the current producing it and, in this case, the "force" being that which actuates the diaphragm and the "current" being that which flows in the coil. According to an article, The Ring Armature Telephone Receiver, by Mott et al. in the Bell System Technical Journal, Vol. 50, January 1951, pages 110-140, the force factor is a function of the product of the d.c. flux and the a.c. flux.
For the purposes of providing d.c. flux in such auxiliary gap, the last named Mott patent discloses a transducer structure in which the permanent magnet heretofore referred to is in the form of a vertically elongated cylindrical ring magnetically coupled at its top or "N" end to the armature and its bottom or "S" end to the lower pole piece. Further, such "main" permanent magnet is supplemented by an auxiliary permanent magnet in the form of a horizontal annular disc ring disposed above the main magnet with its outer and inner margins being positioned, respectively, above the top of the main magnet, and above the inner margin of the armature to be spaced therefrom by an "auxiliary" gap, the auxiliary magnet being polarized "S" and "N" at, respectively, its outer and inner margins. With the progress of time and as exemplified by the transducer shown in, for example, U.S. Pat. No. 2,506,624 issued May 9, 1950 in the name of R. E. Wirching, the two permanent magnets were structurally combined into a single magnet by eliminating the spacing between them and making them integral with each other, but that structural change had no effect on the magnetic circuit of the transducer for which, magnetically speaking, there is still a main magnet section and an auxiliary magnet section in, respectively, one and the other of the two flux loops of the circuit which pass through the armature. Thus, whether such two loops respectively include two separate magnets or two different sections of a structurally single magnetized element, such circuit is, in a magnetic sense, a dual magnet, dual gap circuit.
As taught in Mott U.S. Pat. No. 2,249,160 and in more detail in the mentioned Mott et al. article, in a ring armature transducer of the sort described with a dual magnet, dual gap magnetic circuit, the radial flow of d.c. flux in the armature from the auxiliary magnet is opposite to that of the d.c. flux from the main magnet, and, by proper selection of magnetic conditions, there can be obtained a full flux balance for which the d.c. flux values in the main and auxiliary gaps are the same, and the oppositely flowing d.c. fluxes in the armature cancel out to yield in theory the advantages of maximizing the force factor and maximizing the dynamic permeability of the armature to thereby minimize its reluctance to a.c. flux. In practice, however, it is not possible to utilize such a full balance because of another consideration, namely that, in order for the armature to have adequate positional stability, it has to be biased downward by magnetic attractive force. Hence, to obtain such magnetic bias while still realizing as much as possible the benefits of magnetic balancing of the d.c. flux, it was settled upon to provide a weaker magnetic field in the auxiliary gap than in the main gap such that a 25% to 50% imbalance exists between the respective fluxes in those two gaps.
Recapitulating now the differences between the single magnet single gap circuit of Mott's '571 patent and the dual magnet dual gap circuit of his '160 patent, the single gap circuit of the former patent had 100% imbalance of d.c. fluxes to thus be better than necessary in providing positional stability of the armature. Because, however, of the absence of the extra magnetomotive force ("MMF") from an auxiliary magnet and the fact that the d.c. flux in the armature was equal to the full d.c. flux in the circuit to thereby bias the armature at a point on its saturation curve yielding high dynamic reluctance of the armature to a.c. flux, the single gap circuit had a poor force factor compared to the later dual magnet dual gap circuit of the Mott '160 patent.
As an alternative to both such circuits, Mott proposed in a single paragraph in his U.S. Pat. No. 2,566,849, issued Sept. 4, 1951, a ring armature transducer with a single magnet double gap circuit arrived at, in essence, by adding to his previous single magnet single gap circuit an unpolarized magnetic annular disc member of, say, "Permalloy" disposed over the ring armature such that the outer margins of the armature and member were magnetically coupled and the inner margins thereof were separated by an auxiliary gap complementing the main gap between the armature and lower pole piece. In a magnetic sense, that disc member added to the single magnet single gap circuit a magnetic shunt for the flux path through the armature. The effect of the shunt was to render the reluctance between the magnet and the main gap the combined reluctance of two parallel branches consisting of (1) the armature and (2) the unpolarized disc member and the auxiliary gap in series. Such combined reluctance would of course be lower than that of the armature alone to thereby produce the effects of an increase in the d.c. flux through the main gap, and of a splitting of the d.c. flux flowing from the magnet to the main gap into separate fractions carried by the member and armature, the fraction of such flux flowing through the armature being less than the d.c. flux it would carry if such magnetic shunt were not present. Because, however, the shunt flux path paralleling that of the armature included not only the relatively low reluctance of the disc member but also the high reluctance of the auxiliary gap, the mentioned effects would be relatively small in magnitude. Thus, as compared to the single magnet single gap circuit of the Mott '571 patent, the single magnet double gap circuit proposed in Mott's '849 patent would in theory be only a minor improvement and, as compared to the double magnet double gap circuit disclosed in the Mott '160 patent, the proposed circuit would in theory and in an overall sense be retrogressive in that while providing a flux imbalance likely more than necessary to assure adequate stability of the armature, it would, due to the absence of an auxiliary magnet, and due also to the relatively high level of d.c. flux through the armature (leading to its high dynamic reluctance to a.c. flux), have a substantially poorer force factor than a comparable double magnet double gap circuit.
Perhaps, for the reasons stated above, ring armature transducers having a single magnet double gap circuit of the sort proposed in the Mott '849 patent have never, insofar as we know, been put to practical use. Instead, the double magnet double gap design for the magnetic circuit has been the one popularly selected for ring armature transducers as exemplified, for example, by the transducers disclosed in U.S. Pat. No. 4,075,437 issued Feb. 21, 1978 in the name of Chin et al., U.S. Pat. No. 4,258,234 issued Mar. 24, 1981 in the name of Bordelon et al., and U.S. Patent Application Ser. No. 262,602 filed May 11, 1981 in the name of Bordelon et al., and assigned to the assignee hereof, such two last named patents and such patent application being incorporated herein by reference.
Until recently, the permanent magnetic material used in ring armature transducers has been either the aluminum-nickel cobalt alloy known commercially as Alnico or the molybdenum-cobalt iron alloy known commercially as Remalloy. Within the past few years, however, there has been developed a family of Fe-Co-Cr magnetic alloys which are known commercially as Chromindur alloys and which are disclosed, for example, in the mentioned patents to Chin et al. and to Bordelon et al. Chromindur alloys have been employed in lieu of Alnico or Remalloy to provide, as taught by those patents, the permanent magnets utilized in ring armature electroacoustic transducers. The representative embodiments of transducers disclosed by these patents have, however, dual magnet dual gap magnetic circuits which, as stated, have in the past offered overall more advantages in practical use than have other types of magnetic circuits.
There has developed, however, a problem in the extensive use of Chromindur alloys in transducers of the ring armature type. Specifically, the cobalt and chromiun constituents of the alloy, have become very expensive. Moreover, both chromium and cobalt are in limited supply so as to render incertain the degree to which those metals will be available in the future.
Accordingly, it would clearly be desirable to provide ring armature electroacoustic transducers which, while taking advantage of the superior magnetic qualities of Chromindur alloys, use less of it disclosed in the mentioned Chin et al. and Bordelon et al. patents, and which, at the same time, perform from the magnetic circuit viewpoint in a manner which is fully adequate to meet commercial needs.